The present invention relates generally to frequency modulated (FM) signal transmission, and more particularly to modulating circuitry for FM transmitters.
In communication systems, it is becoming increasingly necessary to communicate binary signals between stations. Such binary signals are typically encoded in FM communication systems by symmetrically deviating the frequency of the transmitted FM signal. For example, the frequency of the FM signal may be increased by a predetermined amount in response to a first binary state and decreased by the same predetermined amount in response to a second binary state. However, if the transmitted binary signal does not have an equal number of each of the binary states, it is possible that the frequency of the transmitted FM signal will be offset by a small amount. Because of the frequency offset, the frequency of the FM signal will be deviated unsymmetrically by the binary signal. The frequency stability of the transmitter appears to be degraded by such frequency offsets. Both degradation in the frequency stability and maximum frequency deviation are strictly regulated by the Federal Communications Commission. Furthermore, since the receiving station is precisely tuned to the predetermined frequency of the RF signal, reception of the modulated RF signal is also degraded due to the frequency offset introduced by the binary signal. The introduction of such frequency offsets is further aggravated as the bit rate of the binary bit stream is decreased.
One solution to the frequency offset problem is to directly modulate the transmitter oscillator which may be thereafter multiplied to generate the RF signal. However, a modulatable oscillator not only costs more than, but also has a lower frequency stability than non-modulatable oscillators.
In the case of frequency synthesized transmitters, symmetrical deviation of the FM signal can be obtained by both modulating the reference oscillator and the voltage-controlled oscillator in the frequency synthesizer. However, such frequency synthesized transmitters must likewise utilize expensive modulatable oscillators in order to achieve high frequency stability.
Another approach utilizing a synthesizer modulates the voltage-controlled oscillator with voice signals and the loop divider with binary signals. However, such an approach requires a wider loop bandwidth when the loop divider is modulated with a binary signal. As the loop bandwidth is increased to accomodate binary signals, noise and distortion of the modulated signal are more difficult to simultaneously minimize.
According yet to another approach, two frequency synthesizers can be utilized, one for binary signal modulation and one for voice signal modulation. In such transmitters, the voice signal modulates the voltage-control oscillator of its synthesizer and the binary signal modulates the divider of its synthesizer. The modulated signals from the two frequency synthesizers may then be mixed together to provide a composite RF signal. The two synthesizer approach is not only costly in that two synthesizers are required, but also is subject to the same noise and distortion problem encountered in the preceding approach.
Thus, none of the foregoing prior art approaches can provide an inexpensive frequency modulator that symmetrically deviates an RF signal with either voice or binary signals without introducing undesirable frequency stability, noise or distortion degradation.